Components of high temperature turbine engines are often manufactured from nickel-, cobalt-, or iron-based superalloy materials, which are recognized as providing greater shape retention and strength retention over a wider range of operating temperatures than other candidate materials for these applications. Although superalloy materials exhibit improved mechanical properties at high operating temperatures, they are nonetheless susceptible to high temperature oxidation, hot corrosion, and stress corrosion cracking. While the efficiency of a turbine engine generally increases with increasing operating temperature, the ability of superalloy materials to operate at such increased temperatures is limited by the ability to withstand such oxidation and corrosion.
Generally, gas turbine engines, such as jet engines and industrial gas turbine engines, include a compressor, with shaft-mounted compressor blades, for compressing incoming air, a combustor for mixing the compressed air with fuel, such as jet fuel or natural gas, and igniting the mixture, and a turbine section, including stationary vanes and rotating turbine blades mounted on the same shaft to drive the compressor. An additional turbine shaft from a second turbine section can drive a fan in a jet engine or a power generator in an industrial gas turbine engine. In particular, gas turbine engines operate by drawing air into the front of the engine. The air is then compressed, mixed with fuel, and combusted. Hot combustion gases from the combusted mixture expand through the turbine, rotating the turbine blades and thereby powering the compressor. The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion and exhaust gas temperatures. The maximum temperature of the combustion gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine, upon which the hot combustion gases impinge. In current engines, the turbine vanes and blades are typically made of nickel-based superalloys.
External surfaces of superalloy turbine engine components, which may experience direct contact with the hot combustion gases, are susceptible to high temperature oxidation and hot corrosion that accelerates the oxidation process. These external surfaces are frequently provided with an intermetallic or aluminide overlayer or diffusion coating that protects the underlying superalloy material against high temperature oxidation and hot corrosion by forming a stable thermal oxide scale. High temperature oxidation and hot corrosion, if the temperature is sufficiently high, may form corrosive deposits which attack and degrade the protective oxide scale.
In addition to the oxidation and corrosion brought on by exposure to elevated temperatures, vane segments or buckets of the turbine engines are subject to extreme cyclic stresses. The flow of the gases through the engine creates variation in pressure in the engine that causes cyclic flexing of each vane. Due to the high rotor assembly rotation rates, vane segments are subject to continuous cyclic stresses that cause cracks to appear in the surface of the vane segment. This is called stress corrosion cracking. Generally, for vane segments, cracks often appear on the trailing edges and in the fillet radius of the vane segment where it connects to the buttress walls.
These cracks are repaired where possible because of the significant cost associated with manufacturing new turbine engine components. In general, the repair process includes cleaning the component and then filing the cracks with a brazing compound. Specifically, the protective coatings are initially stripped from the incoming turbine engine component. Following stripping, it is customary to tumble the parts to remove any residual material or so-called “smut” from the component prior to rinsing the component. Once completely stripped, the parts are submitted to Fluoride Ion Cleaning (FIC), usually in a dynamic FIC system, which relies on the high reactivity of fluorine or fluoride ions for cleaning the exterior surface of the component. FIC cleaning is an extremely hazardous and environmentally unfriendly method of cleaning turbine engine components. In this process, hydrofluoric acid (HF) gas is heated in a retort chamber. The vaporized HF gas attacks any oxides present on the surface of the stripped component. The fluoride ion dissolves any oxides, usually in the form of a spinel, from inside the cracks.
Following FIC cleaning, the components are removed from the furnace and immediately placed into a vacuum furnace. The volatile fluorides are vacuumed off by heating them to a temperature where they are either liquid or gas and then subjecting the components to a vacuum of 10−5 Torr or 10−6 Torr to remove the liquid or gas residue.
Following vacuum cleaning, any cracks are filled with a braze alloy that closely approximates the chemical makeup of the alloy. The brazing alloy composition is formulated such that it contains a eutectic melting constituent called “low melt.” In other words, the melting temperature of the brazing alloy is lower than the melting temperature of the component being joined.
As is known in the art, brazing is a metal-joining process in which two or more metallic parts are joined together by heating a brazing metal or alloy in contact with the parts to be joined. The brazing alloys generally melt above about 450° C. (about 840° F.) but below the melting temperature of the part. These temperatures are higher than those normally encountered in soldering operations. The molten brazing alloy is distributed between two or more close-fitting metal parts often by capillary action. The molten brazing alloy wets the components and may alloy with a thin layer of the bulk metal of the components.
In the case of crack repair, subsequent heat treatment of the braze-filled crack allows the brazing compound to flow within the crack and alloy with the surrounding bulk metal, essentially healing the crack. Wetting of the component by the brazing alloy is necessary to form a metallic bond between the brazing alloy and the part. However, poor cleaning or preparation of the component, including the region of the crack may inhibit wetting of the crack surfaces with the molten brazing compound resulting in an unbonded or a weakly bonded crack.
As is known in the art, chemical characteristics of the part's surface directly affect the quality of the braze joint. For instance, poor surface chemistry and/or surface contamination can negatively impact the strength of the braze joint because the braze alloy may not wet the surface to the degree necessary to form a strong joint. “Wetting” refers to the tendency of a molten material to spontaneously spread along a surface. To improve consistency and quality of braze joints, a coating of metal that is known to allow the molten braze alloy to wet it and thereby improve the wetting of the molten braze alloy with the metal part is often placed onto the part prior to brazing.
These pre-brazing coatings are often deposited by electroplating techniques. Electroplating requires a plating solution that contains the metal that is to be plated. These solutions are typically composed of multiple chemicals that must be monitored and adjusted during operation to obtain consistent quality platings. Accordingly, the complexity of the solution often leads to process control issues that ultimately results in inconsistent quality coatings. In addition, as is known in the art, plating operations often produce harmful or toxic byproducts and/or waste streams, like depleted plating solution, that require special handling and for which there are stringent disposal standards.
Consequently, there is a need for method of preparing metallic components for subsequent brazing in a manner that is more environmentally friendly and that includes forming a metallic wetting layer that is more consistent.